Method for manufacturing noble metal fine particles

ABSTRACT

A method for manufacturing noble metal fine particles, by which noble metal fine particles are obtained whose particle diameter and alloy composition are easy to control and whose crystallinity and purity are high, is provided. The method includes the steps of:
     (1) obtaining a molten mixture containing a noble metal chloride, by insufflating chlorine gas into a mixture of a molten salt of an alkali metal chloride and a noble metal;   (2) obtaining a noble metal oxide as a precipitate by adding an alkali metal carbonate to the molten mixture under an inert gas atmosphere;   (3) obtaining a mixture containing noble metal oxide fine particles, by pulverizing the noble metal oxide with at least one of an alkali metal carbonate and an alkali earth metal carbonate; and   (4) obtaining noble metal fine particles by heating the mixture obtained in step (3) under an atmosphere of gas containing hydrogen, and then treating the heat-treated mixture with acid.

TECHNICAL FIELDS

The present invention relates to a method for manufacturing noble metal fine particles, and in particular, to a method for manufacturing fine particles of rhodium, platinum, palladium, and alloy fine particles formed from two or more of the same.

BACKGROUND ART

Recently, a demand is expanding for fine particles of noble metals, such as platinum, rhodium, palladium, and the like, as materials that are essential as highly fine electrode materials of high-performance electronic parts and as electrode catalysts of high-power fuel cells. Therefore, enhancement of performance of the noble metal fine particles is also desired.

There are two methods for manufacturing noble metal fine particles, i.e., a breakdown method and a bottom-up method.

In the breakdown method, oxides of noble metals, inorganic salts of noble metals, organic compounds containing noble metals, and the like are used as noble metal materials. The noble metal materials are mechanically pulverized and then reduced to obtain noble metal fine particles.

Specifically, Japanese Unexamined Patent Publication Nos. 1998-102104, 1998-102106, and 1998-102107 disclose methods for manufacturing platinum powder, rhodium powder, or platinum/rhodium alloy powder, in which platinum compound powder, rhodium compound powder, or the mixture thereof is mixed with calcium carbonate powder and reduced by being heated at 800 to 1400° C., thereby causing particle growth. Further, Japanese Unexamined Patent Publication Nos. 2003-277802 and 2003-277812 disclose methods for manufacturing platinum powder or rhodium powder, in which rhodium compound powder or platinum compound powder is mixed with alkali metal compound powder and alkali earth metal compound powder and heated at 1500° C. or lower under a non-oxygen gas atmosphere, thereby causing particle growth. However, in any of the above methods, the obtained noble metal particles have an unduly large particle diameter because noble metal compound particles have a large particle diameter. In addition, in any of the above methods, improvements in the crystallinity and the purity of the noble metal particles cannot be expected because the crystallinity and the purity of the noble metal particles depend on the crystallinity and the purity of the noble metal compound particles. Further, noble metal alloy particles obtained from the mixture powder of two or more types of noble metal compound particles tend to have a non-uniform composition in each particle, and the uniformity of the composition cannot be obtained by any of the above methods.

In the bottom-up method, inorganic salts of noble metals, organic compounds containing noble metals, and the like are used as noble metal materials. Then, the noble metal materials are reduced inorganic solvents, such as polyols and the like, to obtain noble metal fine particles. Specifically, Japanese Unexamined Patent Publication Nos. 2006-241494, 2007-19055, and 2006-124787 disclose methods for manufacturing silver nanoparticles with a size of 50 to 100 nm and the slurry thereof. However, in the manufacturing methods, it is possible to manufacture noble metal nanoparticles with a size of 100 nm or smaller, but it is difficult to manufacture alloy fine particles with a relatively large size of 100 nm or greater.

DISCLOSURE OF THE INVENTION

An objective of the present invention is to provide a method for manufacturing noble metal fine particles, by which noble metal fine particles are obtained whose particle diameter and alloy composition are easy to control and whose crystallinity and purity are high.

The inventors of the present invention have found, as a result of thorough research for achieving the above objective, that a method for manufacturing noble metal fine particles, which includes specific steps, can achieve the above objective, and have completed the present invention.

Specifically, the present invention relates to the following methods for manufacturing noble metal fine particles.

1. A method for manufacturing noble metal fine particles, the method comprising the steps of:

(1) obtaining a molten mixture containing a noble metal chloride, by insufflating chlorine gas into a mixture of a molten salt of an alkali metal chloride and a noble metal;

(2) obtaining a noble metal oxide as a precipitate by adding an alkali metal carbonate to the molten mixture under an inert gas atmosphere;

(3) obtaining a mixture containing noble metal oxide fine particles, by pulverizing the noble metal oxide with at least one of an alkali metal carbonate and an alkali earth metal carbonate; and

(4) obtaining noble metal fine particles by heating the mixture obtained in step (3) under an atmosphere of gas containing hydrogen, and then treating the heat-treated mixture with acid.

2. The method according to Item 1, wherein in step (1), a molten mixture A and a molten mixture B, which contain different noble metals, are prepared and mixed together to obtain a molten mixture C.

3. The method according to Item 1 or 2, wherein the noble metal is at least one member selected from the group consisting of rhodium, platinum, and palladium.

4. The method according to any one of Items 1 to 3, wherein the alkali metal chloride is at least one member selected from the group consisting of LiCl, NaCl, KCl, CsCl, LiCl—NaCl, LiCl—KCl, NaCl—KCl, NaCl—CsCl, LiCl—CsCl, KCl—CsCl, LiCl—KCl—NaCl, LiCl—MgCl₂, LiCl—CaCl₂, LiCl—BaCl₂, NaCl—MgCl₂, NaCl—CaCl₂, NaCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, CsCl—MgCl₂, CsCl—CaCl₂, and CsCl—BaCl₂.

5. The method according to any one of Items 1 to 4, wherein in step (1), the mixture ratio of the noble metal and the alkali metal chloride is 1:5 to 20 in weight ratio.

6. The method according to any one of Items 1 to 5, wherein in step (1), the insufflated amount of the chlorine gas is set such that the mole ratio of the chlorine gas to the noble metal is 1:1.5 to 5.

7. The method according to any one of Items 1 to 6, wherein in step (2), the added amount of the alkali metal carbonate is set such that the mole ratio of the alkali metal carbonate to the noble metal chloride is 1:1 to 10.

8. The method according to any one of Items 1 to 7, wherein in step (3), the noble metal oxide is pulverized so as to have an average particle diameter of 10 nm to 10 μm.

9. The method according to any one of Items 1 to 8, wherein said at least one of the alkali metal carbonate and the alkali earth metal carbonate, which are used in step (3), is at least one member selected from the group consisting of sodium carbonate, magnesium carbonate, calcium carbonate, and barium carbonate.

The method for manufacturing noble metal fine particles according to the present invention is characterized by comprising the steps of:

(1) obtaining a molten mixture containing a noble metal chloride, by insufflating chlorine gas into a mixture of a molten salt of an alkali metal chloride and a noble metal;

(2) obtaining a noble metal oxide as a precipitate by adding an alkali metal carbonate to the molten mixture under an inert gas atmosphere;

(3) obtaining a mixture containing noble metal oxide fine particles, by pulverizing the noble metal oxide with at least one of an alkali metal carbonate and an alkali earth metal carbonate; and

(4) obtaining noble metal fine particles by heating the mixture obtained in step (3) under an atmosphere of gas containing hydrogen, and then treating the heat-treated mixture with acid.

In the method for manufacturing noble metal fine particles according to the present invention, which includes the above feature, in particular, because the molten mixture containing the noble metal chloride is obtained in step (1), noble metal fine particles are obtained whose particle diameter and alloy composition are easier to control and whose crystallinity and purity are higher than that obtained by a conventional method. Particularly, in the case where, in step (1), a plurality of molten mixtures (e.g. the molten mixture A and the molten mixture B) containing different noble metals are prepared and mixed together in a desired proportion, the composition of obtained noble metal alloy fine particles is easy to control.

The individual steps of the method for manufacturing noble metal fine particles according to the present invention are explained in detail below.

<<Step 1>>

In step (1), a molten mixture containing a noble metal chloride is obtained by insufflating chlorine gas into a mixture of a molten salt of an alkali metal chloride and a noble metal.

The noble metal is, for example, preferably one selected from the group consisting of rhodium, platinum, palladium. Specifically, simple metals, such as rhodium metal, platinum metal, palladium metal, and the like, and the alloys of these metals can be used. Types of the simple metals and the alloys are selected depending on types and compositions of noble metal fine particles that are target products. The simple metals and the alloys of the noble metals may be in powder form or in bulk form.

The alkali metal chloride is, for example, preferably one selected from the group consisting of LiCl, NaCl, KCl, CsCl, LiCl—NaCl, LiCl—KCl, NaCl—KCl, NaCl—CsCl, LiCl—CsCl, KCl—CsCl, LiCl—KCl—NaCl, LiCl—MgCl₂, LiCl—CaCl₂, LiCl—BaCl₂, NaCl—MgCl₂, NaCl—CaCl₂, NaCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, CsCl—MgCl₂, CsCl—CaCl₂, and CsCl—BaCl₂. Among these alkali metal chlorides, each alkali metal chloride containing two or more components means the mixture of the components, and, for example, NaCl—CsCl means the mixture of NaCl and CsCl. Among these alkali metal chlorides, NaCl—CsCl, KCl—CsCl, etc., are preferable, and the mixture of NaCl and CsCl with a mole ratio of 1:2 and the mixture of KCl and CsCl with a mole ratio of 1:2 are preferable.

The alkali metal chloride may be heated and melted at 600 to 700° C., and a mixture is prepared by mixing the molten salt and the noble metal. Here, the mixture ratio of the noble metal and the alkali metal chloride is preferably 1:5 to 20 in weight ratio.

A molten mixture containing a noble metal chloride is obtained by insufflating chlorine gas into the above mixture. The noble metal chloride is dissolved in the molten salt by the chlorination of the noble metal. Here, the mole ratio of the insufflated amount of the chlorine gas to the noble metal is preferably 1:1.5 to 5.

In the present invention, in the case where the final target products are noble metal alloy fine particles, in addition to use a noble metal alloy as a noble metal material, it is also preferable that a molten mixture A and a molten mixture B, which contain different noble metals, be prepared and mixed together to obtain a molten mixture C. Here, the mixture ratio of the molten mixture A and the molten mixture B are set in accordance with a desired alloy composition. As descried above, when the molten mixture A and the molten mixture B are mixed in a desired ratio to obtain the molten mixture C, the ratio of the noble metal component contained in the molten mixture C can be easily controlled. Thus, the composition of the finally obtained noble metal alloy fine particles is easy to control.

<<Step 2>>

In step (2), a noble metal oxide is obtained as a precipitate by adding an alkali metal carbonate to the molten mixture under an inert gas atmosphere. Examples of inert gas include nitrogen gas, argon gas, nitrogen-argon mixture gas, and the like. Further, examples of the alkali metal carbonate include sodium carbonate, calcium carbonate, and the like, and among them, sodium carbonate is preferable.

The temperature of the molten mixture is not particularly limited as long as a melted state is maintained, but the temperature is preferably about 600 to about 800° C. The amount of the alkali metal carbonate added to the molten mixture is preferably set such that the mole ratio of the alkali metal carbonate to the noble metal chloride is 1:1 to 10.

<<Step 3>>

In step (3), a mixture containing noble metal oxide fine particles is obtained by pulverizing the noble metal oxide with at least one of an alkali metal carbonate and an alkali earth metal carbonate.

The at least one of the alkali metal carbonate and the alkali earth metal carbonate, which are used in step (3), is used as a matrix when the noble metal oxide is pulverized (ground), and also serves advantageously in adjusting the particle diameter of noble metal fine particles that are target products.

Examples of the at least one of the alkali metal carbonate and the alkali earth metal carbonate include sodium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, and the like. Among them, calcium carbonate is preferable. The average particle diameter of the metal carbonate before the pulverization is not particularly limited, but the metal carbonate is preferably in granular form in view of handleability.

The amount of the matrix added to the noble metal oxide is preferably set such that the weight ratio of the matrix to the noble metal oxide is 1:1 to 10.

In order to pulverize the noble metal oxide together with the matrix, a pulverizer, such as a well-known mortar grinding machine, a planetary ball mill, a ball mill, or an attritor, may be used.

The average particle diameter of the noble metal oxide fine particles after the pulverization is preferably about 10 nm to about 10 μm.

By the pulverization, the noble metal oxide fine particles are obtained generally in the following state. The carbonate, which is the matrix, is pulverized and aggregates to form secondary particles with a size of about several hundred μm, and the noble metal oxide fine particles are obtained in the state where the noble metal oxide fine particles with an average particle diameter of about 10 nm to about 10 μm are supported on the surfaces of the secondary particles.

<<Step 4>>

In step (4), noble metal fine particles are obtained by heating the mixture obtained in step (3) under an atmosphere of gas containing hydrogen, and then treating the heat-treated mixture with acid.

The gas containing hydrogen has no limitations as long as it contains 1 to 100 volume % hydrogen, and is preferably a reducing mixture gas of hydrogen and inert gas. The heating temperature is preferably 1000 to 1500° C., and the noble metal oxide fine particles are reduced to be transformed into noble metal fine particles by being heated under an atmosphere of such reducing gas. Further, the matrix used in step (3) is decomposed into a metal oxide (calcium oxide, sodium oxide, magnesium oxide, barium oxide, and the like) and carbon dioxide by the heat treatment, and the noble metal fine particles are supported on the generated metal oxide.

Following that, the particle diameter and the size of the noble metal fine particles supported on the metal oxide can be adjusted by maintaining the heat treatment at about 1000 to about 1500° C. (aging). For example, in the case where rhodium fine particles are supported on calcium oxide, the rhodium fine particles do not aggregate and extremely small particles on the calcium oxide surfaces disappear by aging, and particles with a relatively large particle diameter grow larger in size by particle growth and form a spherical shape. This is because noble metal fine particles with a smaller particle diameter have greater surface free energy, and hence are chemically unstable and absorbed by large particles (Ostwald Ripening). Thus, the time duration for the heat treatment is not particularly limited as long as the noble metal oxide fine particles are reduced to be transformed into the noble metal fine particles, and the time duration is adjusted as appropriate, for example, in a range of 0.5 to 20 hours depending on a desired particle diameter. In other words, the particle diameter, the shape, and the like of the noble metal fine particles, which are the target products, can be adjusted within the desired range by pulverizing the noble metal oxide into small particles as much as possible in step (3), and conducting aging treatment in step (4).

After the heat treatment, desired noble metal fine particles are obtained by treating the heat-treated object with acid. By the acid treatment, the metal oxide (i.e., the above matrix powder) such as calcium oxide is dissolved and removed. For the acid treatment, hydrochloric acid, sulfuric acid, nitric acid, and the like can be used. Further, washing with water is conducted in combination according to need. Desired noble metal fine particles are obtained through the above steps.

In the method for manufacturing noble metal fine particles according to the present invention, in particular, because the molten mixture containing the noble metal chloride is obtained in step (1), noble metal fine particles are obtained whose particle diameter and alloy composition are easier to control and whose crystallinity and purity are higher than that obtained by a conventional method. Particularly, in the case where, in step (1), a plurality of molten mixtures (e.g., the molten mixture A and the molten mixture B) containing different noble metals are prepared and mixed together in a desired proportion, the composition of obtained noble metal alloy fine particles is easy to control.

The noble metal fine particles whose particle diameter and alloy composition are controlled are suitably used as highly fine electrode materials of high-performance electronic parts and electrode catalysts of high-power fuel cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an XRD pattern of Pt/Rh alloy fine particles (example 3);

FIG. 2 shows an SEM image of the Pt/Rh alloy fine particles (example 3);

FIG. 3 shows an SEM image of Pt/Pd/Rh alloy fine particles (example 4);

FIG. 4 shows an XRD pattern of Pt/Pd alloy fine particles (example 5); and

FIG. 5 shows an SEM image of the Pt/Rh alloy fine particles (example 5).

BEST MODE FOR CARRYING OUT THE INVENTION

The following will describe in detail the present invention with reference to Examples and Comparative Examples. However, the present invention is not limited to the Examples.

Example 1

3 g of metal Rh powder and a 35 mol % NaCl-65 mol % CsCl mixture were mixed in a weight ratio of 1:10. After the mixture was melted at 650° C. under an atmosphere of Ar gas to obtain a molten salt mixture containing the Rh powder, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 3 times of the equivalent amount of the Rh powder while the molten salt mixture was maintained at 650° C., to generate an Rh chloride.

Next, the molten salt mixture containing the Rh chloride was melted at 650° C. under an atmosphere of Ar gas. Then, while the molten salt mixture was maintained at 650° C., Na₂CO₃ was added to the molten salt mixture in the amount corresponding to 1.5 times of the equivalent amount of the Rh chloride, and bubbling was conducted with Ar gas for 1 hour to mix the mixture. Then, the mixture was cooled and washed with water, and an Rh oxide was collected as a black precipitate.

Next, the Rh oxide was mixed with CaCO₃ whose amount was 3 times greater than that of the Rh oxide in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Rh oxide powder with a size of 100 to 1000 nm, which was dispersed in a CaCO₃ matrix. The mixture powder was heated (reduced) at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Rh fine particles.

From the result of SEM (scanning electron microscope) observation, the particle diameter of the Rh fine particles was 0.1 to 2 μm. Further, from the result of XRD (X-ray diffraction) analysis, it was confirmed that the Rh fine particles had high crystallinity. The purity of the Rh fine particles was 99.99% or higher.

Example 2

3 g of a metal Pt piece and a 35 mol % NaCl-65 mol % CsCl mixture were mixed in a weight ratio of 1:10. After the mixture was melted at 650° C. under an atmosphere of Ar gas to obtain a molten salt mixture containing the Pt piece, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 4 times of the equivalent amount of the Pt piece while the molten salt mixture was maintained at 650° C., to generate a Pt chloride.

Next, the molten salt mixture containing the Pt chloride was melted at 650° C. under an atmosphere of Ar gas. Then, while the molten salt mixture was maintained at 650° C., Na₂CO₃ was added to the molten salt mixture in the amount corresponding to 2 times of the equivalent amount of the Pt chloride, and bubbling was conducted with Ar gas for 1 hour to mix the mixture. Then, the mixture was cooled and washed with water, and a Pt oxide was collected as a black precipitate.

Next, the Pt oxide was mixed with CaCO₃ whose amount was 3 times greater than that of the Pt oxide in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt oxide powder with a size of 100 to 1000 nm, which was dispersed in a CaCO₃ matrix. The mixture powder was heated (reduced) at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt fine particles.

From the result of SEM observation, the particle diameter of the Pt fine particles was 0.1 to 2 μm. Further, from the result of XRD analysis, it was confirmed that the Pt fine particles had high crystallinity. The purity of the Pt fine particles was 99.99% or higher.

Example 3

1.6 g of metal Rh powder and a 35 mol % NaCl-65 mol % CsCl mixture were mixed in a weight ratio of 1:10. After the mixture was melted at 650° C. under an atmosphere of Ar gas to obtain a molten salt mixture containing the Rh powder, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 3 times of the equivalent amount of the Rh powder while the mixture was maintained at 650° C., to generate an Rh chloride.

Similarly, 2.4 g of a metal Pt piece and the molten salt mixture was mixed in a weight ratio of 1:10. After the molten salt mixture was melted at 650° C. under an atmosphere of Ar gas, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 4 times of the equivalent amount of the Pt piece while the molten salt mixture was maintained at 650° C., to generate a Pt chloride.

Next, the Pt chloride and the Rh chloride were mixed in a weight ratio of 6:4. The molten salt mixture containing these chlorides was melted at 650° C. under an atmosphere of Ar gas. Then, while the molten salt mixture was maintained at 650° C., Na₂C0₃ was added to the molten salt mixture in the amount corresponding to the sum of 2 times of the equivalent amount of the Pt chloride and 1.5 times of the equivalent amount of the Rh chloride, and bubbling was conducted with Ar gas for 1 hour to mix the mixture. Then, the mixture was cooled and washed with water, and a Pt/Rh oxide was collected as a black precipitate.

Next, the Pt/Rh oxide was mixed with CaCO₃ whose amount was 3 times greater than that of the Pt/Rh oxide in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt/Ph oxide powder with a size of 100 to 1000 nm, which was dispersed in a CaCO₃ matrix. The mixture powder was heated (reduced) at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt/Rh alloy fine particles.

From the result of XRD analysis as shown in FIG. 1, it was confirmed that the Pt/Rh alloy fine particles were Pt/Rh alloy fine particles that had a certain composition and high crystallinity and in which two phases coexist so as to be in thermodynamical equilibrium. From the result of SEM observation as shown in FIG. 2, the particle diameter of the Pt/Rh alloy fine particles was 0.1 to 2 μm. The purity of the Pt/Rh alloy fine particles was 99.99% or higher.

Example 4

0.4 g of metal Rh powder and a 35 mol % NaCl-65 mol % CsCl mixture were mixed in a weight ratio of 1:10. After the mixture was melted at 650° C. under an atmosphere of Ar gas to obtain a molten salt mixture containing the Rh powder, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 3 times of the equivalent amount of the Rh powder while the molten salt mixture was maintained at 650° C., to generate an Rh chloride.

Similarly, 1.8 g of a metal Pt piece and the molten salt mixture were mixed in a weight ratio of 1:10. After the molten salt mixture was melted at 650° C. under an atmosphere of Ar gas, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 4 times of the equivalent amount of the Pt piece while the molten salt mixture was maintained at 650° C., to generate a Pt chloride.

Similarly, 1.8 g of a metal Pd piece and the molten salt mixture were mixed in a weight ratio of 1:10. After the molten salt mixture was melted at 650° C. under an atmosphere of Ar gas, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 2 times of the equivalent amount of the Pd piece while the molten salt mixture was maintained at 650° C., to generate a Pd chloride.

Next, the Pt chloride, the Pd chloride, and the Rh chloride were mixed in a weight ratio of 4.5:4.5:1. The molten salt mixture containing these chlorides was melted at 650° C. under an atmosphere of Ar gas. Then, while the molten salt mixture was maintained at 650° C., Na₂CO₃ was added to the molten salt mixture in the amount corresponding to the sum of 2 times of the equivalent amount of the Pt chloride, the equivalent amount of the Pd chloride, and 1.5 times of the equivalent amount of the Rh chloride, and bubbling was conducted with Ar gas for 1 hour to mix the mixture. Then, the mixture was cooled and washed with water, and a Pt/Pd/Rh oxide was collected as a black precipitate.

Next, the Pt/Pd/Rh oxide was mixed with CaCO₃ whose amount was 3 times greater than that of the Pt/Pd/Rh oxide in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt/Pd/Rh oxide powder with a size of 100 to 1000 nm, which was dispersed in a CaCO₃ matrix. The mixture powder was heated (reduced) at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt/Pd/Rh alloy fine particles.

From the result of SEM observation as shown in FIG. 3, the particle diameter of the Pt/Pd/Ph alloy fine particles was 0.1 to 2 μm. The purity of the Pt/Pd/Rh alloy fine particles was 99.99% or higher.

Example 5

5.1 g of metal Rh powder and a 35 mol % NaCl-65 mol % CsCl mixture were mixed in a weight ratio of 1:10. After the mixture was melted at 650° C. under an atmosphere of Ar gas to obtain a molten salt mixture containing the Rh powder, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 4 times of the equivalent amount of the Rh powder while the molten salt mixture was maintained at 650° C., to generate an Rh chloride.

Next, the Rh chloride and a metal Pt piece were mixed in a weight ratio of 2:3, and the molten salt mixture was added to the mixture in a weight ratio of 1:10. After the molten salt mixture was melted at 650° C. under an atmosphere of Ar gas, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 2 times of the equivalent amount of the Pt piece while the molten salt mixture was maintained at 650° C., to generate a Pt/Rh chloride.

Next, while the molten salt mixture was maintained at 650° C., Na₂CO₃ was added to the molten salt mixture in the amount corresponding to the sum of 2 times of the equivalent amount of the Pt chloride and 1.5 times of the equivalent amount of the Rh chloride, and bubbling was conducted with Ar gas for 1 hour to mix the mixture. Then, the mixture was cooled and washed with water, and a Pt/Rh oxide was collected as a black precipitate.

Next, the Pt/Rh oxide was mixed with CaCO₃ whose amount was 3 times greater than that of the Pt/Rh oxide in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt/Rh oxide powder with a size of 100 to 1000 nm, which was dispersed in a CaCO₃ matrix. The mixture powder was heated (reduced) at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt/Rh alloy fine particles.

From the result of XRD analysis as shown in FIG. 4, it was confirmed that the Pt/Rh alloy fine particles had a certain composition. From the result of SEM observation as shown in FIG. 5, the particle diameter of the Pt/Rh alloy fine particles was 0.2 to 2 μm. The purity of the Pt/Rh alloy fine particles was 99.99% or higher.

Example 6

0.5 g of metal Rh powder and a 35 mol % NaCl-65 mol % CsCl mixture were mixed in a weight ratio of 1:10. After the mixture was melted at 650° C. under an-atmosphere of Ar gas to obtain a molten salt mixture containing the Rh powder, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to 4 times of the equivalent amount of the Rh powder while the molten salt mixture was maintained at 650° C., to generate an Rh chloride.

2.25 g of a metal Pt piece, 2.25 g of metal Pd powder, and the molten salt mixture were mixed in a weight ratio of 1:10. After the molten salt mixture was melted at 650° C. under an atmosphere of Ar gas, chlorine gas was insufflated into the molten salt mixture in the amount corresponding to the sum of 2 times of the equivalent amount of the Pt piece and 2 times of the equivalent amount of the Pd piece while the molten salt mixture was maintained at 650° C., to generate a Pt/Pd/Rh chloride.

Next, while the molten salt mixture was maintained at 650° C., Na₂CO₃ was added to the molten salt mixture in the amount corresponding to the sum of two times of the equivalent amount of the Pt chloride, the equivalent amount of the Pd chloride, and 1.5 times of the equivalent amount of the Rh chloride, and bubbling was conducted with Ar gas for 1 hour to mix the mixture. Then, the mixture was cooled and washed with water, and a Pt/Pd/Rh oxide was collected as a black precipitate.

Next, the Pt/Pd/Rh oxide was mixed with CaCO₃ whose amount was 3 times greater than that of the Pt/Pd/Rh oxide in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt/Pd/Rh oxide powder with a size of 100 to 1000 nm, which was dispersed in a CaCO₃ matrix. The mixture powder was heated under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂. Then, the Pt/Pd/Rh oxide was caused to be supported on the CaO surfaces and was reduced, and the resultant metal fine particles were caused to be supported on the CaO surfaces in a distributed manner and was maintained at 1200° C. for 1 hour so as to adjust the shape and the particle diameter of the Pt/Pd/Rh powder by means of an atom diffusion phenomenon. The powder was cooled, dissolved in HCl, and washed with water to obtain Pt/Pd/Rh alloy fine particles.

From the result of SEM observation, the particle diameter of the Pt/Pd/Ph alloy fine particles was 0.2 to 2 μm. The purity of the Pt/Pd/Rh alloy fine particles was 99.99% or higher.

Comparative Example 1

Rh black powder with a particle diameter of 10 to 100 nm was mixed with CaCO₃ whose amount was 3 times greater than that of the Rh black powder in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Rh black fine powder dispersed in a CaCO₃ matrix. The mixture powder was reduced at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Rh powder.

From the result of SEM observation, the particle diameter of the Rh powder was 0.5 to 5 μm. From the result of XRD analysis, the particles of the Rh powder had low crystallinity. The purity of the Rh powder was 99.9%.

Comparative Example 2

Pt black powder with a particle diameter of 10 to 100 nm was mixed with CaCO₃ whose amount was 3 times greater than that of the Pt black powder in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt black fine powder dispersed in a CaCO₃ matrix. The mixture powder was reduced at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt powder.

From the result of SEM observation, the particle diameter of the Pt powder was 0.5 to 5 μm. From the result of XRD analysis, the particles of the Pt powder had low crystallinity. The purity of the Pt powder was 99.9%.

Comparative Example 3

Pd black powder with a particle diameter of 10 to 100 nm was mixed with CaCO₃ whose amount was 3 times greater than that of the Pd black powder in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pd black fine powder dispersed in a CaCO₃ matrix. The mixture powder was reduced at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pd powder.

From the result of SEM observation, the particle diameter of the Pd powder was 0.5 to 5 μm. From the result of XRD analysis, the particles of the Pd powder had low crystallinity. The purity of the Pd powder was 99.9%.

Comparative Example 4

The mixture powder of Pt black powder and Rh black powder each having a particle diameter of 10 to 100 nm was mixed with CaCO₃ whose amount was 3 times greater than that of the mixture powder in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt black fine powder and Rh black fine powder which were dispersed in a CaCO₃ matrix. The mixture powder was reduced at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt/Rh alloy powder.

From the result of SEM observation, the particle diameter of the Pt/Rh alloy powder was 0.5 to 5 μm. From the result of XRD analysis, the particles of the Pt/Rh alloy powder had low crystallinity. The purity of the Pt/Rh alloy powder was 99.9%.

Comparative Example 5

The mixture powder of Pt black powder, Pd black powder, and Rh black powder each having a particle diameter of 10 to 100 nm was mixed with CaCO₃ whose amount was 3 times greater than that of the mixture powder in weight ratio, and then, the mixture was ground by an automatic mortar grinder to obtain Pt black fine powder, Pd black fine powder, and Rh black fine powder which were dispersed in a CaCO₃ matrix. The mixture powder was reduced at 1200° C. for 1 hour under an atmosphere of 4 mol % H₂-96 mol % Ar gas so as to decompose CaCO₃ into CaO and CO₂, and then, the mixture powder was cooled, dissolved in HCl, and washed with water to obtain Pt/Pd/Rh alloy powder.

From the result of SEM observation, the particle diameter of the Pt/Pd/Rh alloy powder was 0.5 to 5 μm. From the result of XRD analysis, the particles of the Pt/Pd/Rh alloy powder had low crystallinity. The purity of the Pt/Pd/Rh alloy powder was 99.9%. 

1. A method for manufacturing noble metal fine particles, the method comprising the steps of: (1) obtaining a molten mixture containing a noble metal chloride, by insufflating chlorine gas into a mixture of a molten salt of an alkali metal chloride and a noble metal; (2) obtaining a noble metal oxide as a precipitate by adding an alkali metal carbonate to the molten mixture under an inert gas atmosphere; (3) obtaining a mixture containing noble metal oxide fine particles, by pulverizing the noble metal oxide with at least one of an alkali metal carbonate and an alkali earth metal carbonate; and (4) obtaining noble metal fine particles by heating the mixture obtained in step (3) under an atmosphere of gas containing hydrogen, and then treating the heat-treated mixture with acid.
 2. The method according to claim 1, wherein in step (1), a molten mixture A and a molten mixture B, which contain different noble metals, are prepared and mixed together to obtain a molten mixture C.
 3. The method according to claim 1, wherein the noble metal is at least one member selected from the group consisting of rhodium, platinum, and palladium.
 4. The method according to claim 1, wherein the alkali metal chloride is at least one member selected from the group consisting of LiCl, NaCl, KCl, CsCl, LiCl—NaCl, LiCl—KCl, NaCl—KCl, NaCl—CsCl, LiCl—CsCl, KCl—CsCl, LiCl—KCl—NaCl, LiCl—MgCl₂, LiCl—CaCl₂, LiCl—BaCl₂, NaCl—MgCl₂, NaCl—CaCl₂, NaCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, CsCl—MgCl₂, CsCl—CaCl₂, and CsCl—BaCl₂.
 5. The method according to claim 1, wherein in step (1), the mixture ratio of the noble metal and the alkali metal chloride is 1:5 to 20 in weight ratio.
 6. The method according to claim 1, wherein in step (1), the insufflated amount of the chlorine gas is set such that the mole ratio of the chlorine gas to the noble metal is 1:1.5 to
 5. 7. The method according to claim 1, wherein in step (2), the added amount of the alkali metal carbonate is set such that the mole ratio of the alkali metal carbonate to the noble metal chloride is 1:1 to
 10. 8. The method according to claim 1, wherein in step (3), the noble metal oxide is pulverized so as to have an average particle diameter of 10 nm to 10 μm.
 9. The method according to claim 1, wherein said at least one of the alkali metal carbonate and the alkali earth metal carbonate, which are used in step (3), is at least one member selected from the group consisting of sodium carbonate, magnesium carbonate, calcium carbonate, and barium carbonate.
 10. The method according to claim 2, wherein the noble metal is at least one member selected from the group consisting of rhodium, platinum, and palladium.
 11. The method according to claim 2, wherein the alkali metal chloride is at least one member selected from the group consisting of LiCl, NaCl, KCl, CsCl, LiCl—NaCl, LiCl—KCl, NaCl—KCl, NaCl—CsCl, LiCl—CsCl, KCl—CsCl, LiCl—KCl—NaCl, LiCl—MgCl₂, LiCl—CaCl₂, LiCl—BaCl₂, NaCl—MgCl₂, NaCl—CaCl₂, NaCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, CsCl—MgCl₂, CsCl—CaCl₂, and CsCl—BaCl₂.
 12. The method according to claim 3, wherein the alkali metal chloride is at least one member selected from the group consisting of LiCl, NaCl, KCl, CsCl, LiCl—NaCl, LiCl—KCl, NaCl—KCl, NaCl—CsCl, LiCl—CsCl, KCl—CsCl, LiCl—KCl—NaCl, LiCl—MgCl₂, LiCl—CaCl₂, LiCl—BaCl₂, NaCl—MgCl₂, NaCl—CaCl₂, NaCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, KCl—MgCl₂, KCl—CaCl₂, KCl—BaCl₂, CsCl—MgCl₂, CsCl—CaCl₂, and CsCl—BaCl₂.
 13. The method according to claim 2, wherein in step (1), the mixture ratio of the noble metal and the alkali metal chloride is 1:5 to 20 in weight ratio.
 14. The method according to claim 3, wherein in step (1), the mixture ratio of the noble metal and the alkali metal chloride is 1:5 to 20 in weight ratio.
 15. The method according to claim 4, wherein in step (1), the mixture ratio of the noble metal and the alkali metal chloride is 1:5 to 20 in weight ratio.
 16. The method according to claim 2, wherein in step (1), the insufflated amount of the chlorine gas is set such that the mole ratio of the chlorine gas to the noble metal is 1:1.5 to
 5. 17. The method according to claim 2, wherein in step (2), the added amount of the alkali metal carbonate is set such that the mole ratio of the alkali metal carbonate to the noble metal chloride is 1:1 to
 10. 18. The method according to claim 3, wherein in step (2), the added amount of the alkali metal carbonate is set such that the mole ratio of the alkali metal carbonate to the noble metal chloride is 1:1 to
 10. 19. The method according to claim 2, wherein in step (3), the noble metal oxide is pulverized so as to have an average particle diameter of 10 nm to 10 μm.
 20. The method according to claim 2, wherein said at least one of the alkali metal carbonate and the alkali earth metal carbonate, which are used in step (3), is at least one member selected from the group consisting of sodium carbonate, magnesium carbonate, calcium carbonate, and barium carbonate. 